Theres a neat little genre of toys called prostate massagers, usually available in the "male" section of your local adult toy store. They're oddly shaped little devices that are specifically designed to stimulate your prostate and move and shift around as your body contracts around them.

They are anal toys and come in a variety of shapes and sizes. The 'original' is called the aneros, but there are a few equally good knockoffs that wont run you the 60-80 bucks that one will. it doesnt vibe naturally, but you could easily tape a bullet to one of the external pieces and the entire thing would buzz away.

great little toys. Good during sex too.